The blasts move underground
This satellite view of the Nevada Test Site shows all the major test areas. The craters on Yucca Flat are clearly visible. Groom Lake (Area 51) is just off the northeast corner of the NTS near Area 15.
After the nuclear tests moved completely underground, air-sampling stations continued to operate to detect any radiation that might be accidentally released. In addition, observers were stationed at off-site locations, including Area 51, to record ground motion caused by subterranean detonations. When shot TIJERAS was fired on 14 October 1970, Donald Bruskert recorded his observations outdoors at Area 51. According to Bruskert, the motion came "in rolling waves" five seconds after detonation, and that there was "no jolt." According to the DOE list of United States Nuclear Tests, the yield of the blast was between 20 and 200 kilotons. The ground motion lasted for 25 seconds. During shot ARTESIA, on 16 December 1970, William Moore felt the shock distinctly inside the Area 51 security building. Donald Bruskert, standing outdoors, described the motion from the "20 to 200" kiloton blast as "questionable."
The proximity of Watertown/Area 51 to the Nevada Test Site helped shield activities at the airstrip under the blanket of security that already surrounded the nuclear proving ground. It also created such operational difficulties as radiation exposure, damage to facilities and equipment, and numerous delays due to evacuation. Air Force, CIA, and civilian contractor personnel at the secret airbase willingly accepted these risks in order to accomplish their mission: to develop advanced aircraft and systems in defense of the United States.